Multicolor electrophotometry of the peculiar object V1357 Cyg=Cyg X-1 in the period 1986 -- 1992

TL;DR

This study presents multicolor photometric observations of Cyg X-1 over several years, revealing complex variability, brightness declines, and a long-term luminosity trend linked to accretion processes.

Contribution

It provides extensive multicolor photometry of Cyg X-1 over 1986-1992, highlighting superimposed effects on the orbital light curve and long-term brightness changes.

Findings

Detection of brightness declines and outbursts.

Observation of long-term brightening followed by decline.

Quantification of brightness decrease during the outburst period.

Abstract

\rm Observations of close binary system (CBS) V1357 Cyg=Cyg X-1 in the $WBVR$ bands are reported. Photometry was carried out at the telescopes located in Kazakhstan, Uzbekistan and Crimea equipped with single-channel $WBVR$-photometer with photon counting. 1358 individual observations in $WBVR$ bands on 202 nights were obtained. The analysis of the photometric data allows to conclude that different photometric effects are superimposed on the orbital light curve of the system. Among them are brightness declines, outbursts with different duration and amplitude, chaotic variability, which sometimes exceeded the ellipsoidal variability. Strong brightness decreases with magnitude $0^m.035 - 0^m.045$ are observed, they are equal to the contribution of accretion disc to the total luminosity of the system. 10 years of photometric monitoring allowed to detect continous brightening of the system, followed by luminosity decline (slow outburst). After the maximum in 1994-96 the decline was equal to 7% in the $W$ band, 4% in the $V$ band, and 2% in the $V$ and $R$ bands. This paper continues the series of works on close systems with X-ray sources. \textbf{Key words}: Close binary systems - luminosity - ellipsoidal variability - accretion disc